Selecting Windows for Energy Efficiency

The thermal efficiency of your windows has a significant impact on the heating and cooling costs of your home. Learn how to understand the differences between windows and how to choose the right one for your particular climate.

Basics

While the physics of heat transfer via windows can get a bit complicated, there are two main processes that determine how well a window will perform when it comes to keeping heat in or out:

Keeping heat in – Measured by a value called the U-factor, this calculates how well a window insulates. When manufacturing windows, U-factor can be lowered by adding multiple panes and adding a layer of argon gas between the panes.

Allowing in solar heat – Infrared wavelengths (i.e. longer wavelengths than visible light) are felt as heat. A value called Solar Heat Gain Coefficient (SHGC) measures what percentage of available solar heat a window allows inside. SHGC is expressed as a number between 0 and 1, with higher numbers letting in more solar heat. SHGC can be lowered by adding so called low-e glass coatings to the proper panes.

In addition to these two, there is a third important number.. This third figure is called Visible Transmittance (VT) and measures the amount of visible light that is allowed through the window. Also expressed as a number between 0 and 1, a higher VT means more light coming through and thus a brighter structure interior.

A diagram illustrating the different thermal effects of windows.

Understanding the NFRC Label

The National Fenestration Rating Council, or NFRC, is a non-profit organization that independently assesses, rates, and labels new windows based on their energy efficiency properties. This allows architects, builders, contractors, home owners, and anyone else to reliably compare products to one another and make an informed decision about their window choice. Look for a label like the following to determine a window has been rated by the NFRC.

A sample NFRC label with descriptions

Frames Matter Too

There are three main types of window frames, with varying insulation properties, maintenance requirements, and costs:

Wood – Wood is an excellent insulator. Well maintained wood looks great aesthetically. Maintenance can be a chore, and wood is generally the most expensive option.

Vinyl – Also a good insulator, maintenance for vinyl windows is easy. Vinyl windows are generally less expensive than wood ones.

Aluminum – Though easily the cheapest option available and easy to maintain, aluminum is a very poor insulator and lets the highest amount of heat or cool escape your home.

In addition, combinations of these materials are available. Vinyl-clad wood, for example, gives the excellent insulating properties of wood while keeping the easy maintenance requirements of vinyl.

Understand Your Climate

When it comes to heating and cooling, the Energy Star program defines four general areas within the United States:

As you might have guessed, different climates have different recommended window types, based on whether heating effects, cooling effects, or a balance of heating and cooling should be prioritized. Where you call on the map will influence what you are looking for when it comes to U-value and SHGC.

The different heating/cooling zones of the United States, as defined by Energy Star

Recommendations

Based on your climate in the map above, Energy Star recommends meeting the following for optimal window efficiency: